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20 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
culature is very clear (BMNH R11956, R8501, RUB 17), passing
through a canal formed by maxilla, lacrimal, jugal, and
perhaps palatine. The maxilla and lacrimal (BMNH R11956)
are both strongly grooved for the neurovascular bundle. In fact,
the dorsal groove on the maxilla extends rostrally within the
floor of the antorbital cavity, ending at a slit-like foramen directed
ventrally into the bone and opposite the internal antorbital
fenestra (BMNH R11956). This slit almost certainly conducted
the maxillary neurovasculature because all of the external
neurovascular foramina are directed toward it (Fig. 7A).
Among Ornithopoda, as in Lesothosaurus diagnosticus, Heterodontosaurus
tucki (BMNH R8 179 [cast of SAM 3371) probably
had a neurovascular canal between maxilla, lacrimal, and
jugal. However, more similar to Hypsilophodon foxii (discussed
below), the neurovascular branches exited more or less separately
from the antorbital cavity rather than from a single foramen.
Better data are available for Hypsilophodon foxii
(BMNH R197, R192, R5862, R2477). The dorsal surface of the
palatine (BMNH R2477) has a clear fossa extending rostrally
up to the choana (Fig. 8A). Unlike Lesothosaurus diagnosticus,
there is no nasal fossa rostral to it, but the postchoanal strut is
grooved dorsally for the nasal capsule. As in L. diagnosticus,
however, there is a very clear neurovascular canal formed by
maxilla, jugal, lacrimal, and a dorsal, laterally arching process
of the palatine (Fig. 8A). The portion of the maxilla forming
the ventral rim of the external antorbital fenestra (supralveolar
lamina) is drawn up dorsally such that the nerves and vessels
would be lodged in a deep groove. As alluded to above, the
neurovasculature exited the antorbital cavity ventrolaterally via
several large foramina (Fig. 8B).
Whereas these more basal ornithopods retain the basic craniofacial
architecture of other archosaurs, the facial and palatal
skeleton in iguanodontian ornithopods becomes highly-transformed.
with reduction of the antorbital cavity and development
of larger dentitions (Weishampel, 1984; se; section on trends
below). Probably above the level of Iguanodon spp. within Iguanodontia,
the palatine assumes a much more vertical orientation
(Fig. 11A; Lambe, 1920; Heaton, 1972; Norman, 1980;
Norman and Weishampel, 1990; Weishampel and Horner,
1990), largely occluding the postnasal fenestra and restricting
the rostral advance of any pterygoideus musculature. The more
basal iguanodontians retaining an antorbital cavity, such as
Camptosaurus dispar (UUVP 5946), C. prestwichii (Galton and
Powell, 1980), and Iguanodon ather-eldensis (BMNH R5764,
R11521), nevertheless resemble more basal ornithischians with
regard to the relationship of the neurovasculature. There is a
deep groove in the dorsal surface of the maxilla which is
formed into a canal by the addition of the jugal and palatine
(the jugal excludes the lacrimal from the canal in at least Iguanodon
atherfieldensis; Fig. 11B). This groove extends in the floor
of the small antorbital cavity before entering the neurovascular
foramen located opposite the antorbital fenestra (as in Lesothosaurus
diagnosticus; see Fig. 1lB for Iguanodon ather-eldensis).
Hadrosaurs have a completely closed external antorbital
fenestra (Weishampel and Horner, 1990), yet retain a shallow
fossa on the palatine and the same basic structure of the neurovascular
canal (e.g., Edmontosaurus regalis, CMN 2289;
Corythosaurus casuarius, AMNH 5338; Hypacrosaurus sp.,
MOR-609-88-9 I)
The evidence provided by the basal thyreophoran Scelidosaurus
harrisonii (BMNH R1111) is particularly important because
many higher thyreophorans are so derived that direct correspondences
are difficult to identify. S. harrisonii resembles
Hypsilophodon foxii in that the caudodorsal surface of the palatine
is excavated into a fossa extending up to the choana and
the postchoanal strut is grooved for the nasal cartilages. Likewise,
very similar to the other ornithischians discussed here,
there is a neurovascular canal formed by maxilla, lacrimal, ju-
gal, and palatine that is carried rostrally as a groove in the floor
of the antorbital cavity, entering the body of the maxilla at a
foramen opposite the antorbital fenestra. Emausaurus ernsti
(Haubold, 1990) is an even more basal thyreophoran, retaining
a much larger antorbital cavity. Although the existence of a
muscular fossa on the palate is unknown, the course of the
maxillary neurovasculature in E. ernsti closely resembles that
in Scelidosaurus harrisonii and other ornithischians.
The basal ceratopsian Psittacosaurus mongoliensis (AMNH
6535; see Sereno, 1987) holds similar importance as a more
basal member of a very specialized group. It also has a shallow
fossa on the dorsolateral surfaces of the palatine and pterygoid
extending rostrally to the choanal margin and a neurovascular
canal surrounded by maxilla, palatine, and jugal. The neurovascular
canal is large and leads to a series of ventral foramina
opening into the buccal cavity.
Saurischians differ in that the maxillary neurovasculature is
not typically enclosed in a canal formed by several bones; a
canal is presumably an ornithischian apomorphy associated
with the tendency to wall in the antorbital cavity (see section
on trends below). Otherwise, some saurischians, such as Plateosaurus
engelhardti, resemble such ornithischians as Lesothosaurus
diagnosticus and Hypsilophodon foxii in the osteological
correlates of the dorsal pterygoideus and maxillary neurovasculature.
Most basal prosauropods (e.g., Thecodontosaurus
antiquus, Kermack, 1984; Anchisaurus polyzelus, Galton, 1976)
are poorly known with regard to the relevant features and the
following discussion focuses mostly on Plateosaurus engelhardti
(principally AMNH 6810; see also Galton, 1984, 1985c,
1990). The dorsolateral surface of the palatine of P. engelhardti
has a postchoanal strut that is strong laterally and weakens rostrodorsally,
delimiting a caudal (muscular) fossa from a flatter,
rostral, nasal area (Fig. 12D). Although Galton (1985~) did not
list the palatine as an attachment site for the dorsal pterygoideus
in P. engelhardti, the fossa compares well with the presumptive
muscular fossa of other archosaurs.
As in ornithischians, there is, within the antorbital cavity, a
dorsal groove on the maxilla that passes rostrally until it enters
a ventral, slit-like foramen opposite the rostral margin of the
internal antorbital fenestra (Fig. 12D); as in Lesothosaurus
diagnosticus, the external neurovascular foramina are all directed
toward this slit (Fig. 12A). The dorsal groove and foramen
apparently are present in Sellosaurus gracilis (Galton,
1985b), and almost identical relationships of the external neurovascular
foramina also are seen in that taxon and Massospondylus
carinatus (Gow et al., 1990). Caudally in Plateosaurus
engelhardti, processes of the lacrimal and palatine approach
each other in the vicinity of their junction with the maxilla and
jugal (Fig. 12D), but do not form the complete bony canal observed
in the ornithischians (although it was probably completed
with soft tissue).
As a whole, sauropods are poorly known with regard to these
features. The palatines and pterygoids of Camarasaurus lentus
(CM 11338; see also Madsen et al., 1995) and Diplodocus longus
(CM 11161) are more or less vertically oriented in the
vicinity of the antorbital cavity, with perhaps a slight concavity
to their caudodorsolateral surfaces (Fig. 13A). The palatine of
Euhelopus zdanskyi has a stronger dorsolateral fossa, undercutting
the postchoanal portion of the bone (Mateer and McIntosh,
1985). Zhang's (1988) reconstruction of the adductor musculature
attaching to this portion of the palate in Shunosaurus lii
seems reasonable, although he carried the muscle rostrally onto
the vomer which is unlikely. Janensch (1935-36) reconstructed
the pterygoideus musculature of Brachiosaurus brancai as extending
over the entire dorsal surface of the palatine to attach
to the antorbital fenestra; however, the palatine of this form
resembles that of Camarasaurus lentus (McIntosh, 1990; Mad-